Capnography is the monitoring of the concentration or partial pressure of carbon dioxide (CO2) in the respiratory gases. It is usually presented as a graph of expiratory CO2 plotted against time. A capnogram is a direct monitor of the inhaled and exhaled concentration or partial pressure of CO2, where CO2 absorbs infra-red radiation and the presence of CO2 in the gas leads to a reduction in the amount of light falling on a sensor.
In a mainstream capnograph, a sample cell is inserted in the airway between the breathing circuit and an endotracheal tube. A lightweight infrared sensor is attached to the airway adapter. The sensor emits infrared light through the adapter windows to a photodetector typically located on the other side of the airway adapter. The light intensity absorbed by the photodetector is a measure of the end tidal CO2.
In a side-stream capnograph, a CO2 sensor is located in an external main unit and a pump aspirates gas samples from the patient's airway through a long capillary tube into the external main unit. The required sampling flow rate may be high (>400 ml.min-1) where optimal gas flow is considered to be 50-200 ml.min-1 to ensure that the capnographs are reliable in both children and adults.